One of the goals of plant genetic engineering is to produce plants with agronomically desirable characteristics or traits. Advances in genetic engineering have provided the requisite tools to transform plants to contain and express foreign genes. The technological advances in plant transformation and regeneration have enabled researchers to take an exogenous polynucleotide molecule, such as a gene from a heterologous or native source, and incorporate that polynucleotide molecule into a plant genome. The gene can then be expressed in a plant cell to exhibit the added characteristic or trait. In one approach, expression of a gene in a plant cell or a plant tissue that does not normally express such a gene may confer a desirable phenotypic effect. In another approach, transcription of a gene or part of a gene in an antisense orientation may produce a desirable effect by preventing or inhibiting expression of an endogenous gene.
Promoters are polynucleotide molecules that comprise the 5′ regulatory elements that play an integral part in the overall expression of genes in living cells. Isolated promoters that function in plants are useful for modifying plant phenotypes through the methods of genetic engineering. The first step in the process to produce a transgenic plant includes the assembly of various genetic elements into a polynucleotide construct. The construct includes a transcribable polynucleotide molecule (gene of interest) that confers a desirable phenotype when expressed (transcribed) in the plant cells by a promoter that is operably linked to the gene of interest. A promoter in a construct may be homologous or heterologous to the gene of interest also contained therein. The construct is then introduced into a plant cell by various methods of plant transformation to produce a transformed plant cell that may be regenerated into a transgenic plant. The promoter controls expression of the gene of interest to which the promoter is operably linked and thus affects the characteristic or trait conferred by the expression of the transgene in plants.
For production of transgenic plants with various desired characteristics, it would be advantageous to have a variety of promoters to provide gene expression such that a gene is transcribed efficiently in the amount necessary to produce the desired effect. The commercial development of genetically improved germplasm has also advanced to the stage of introducing multiple traits into crop plants, often referred to as a gene stacking approach. In this approach, multiple genes conferring different characteristics of interest can be introduced into a plant. It is often desired when introducing multiple genes into a plant that each gene is modulated or controlled for optimal expression, leading to a requirement for diverse regulatory elements. In light of these and other considerations, it is apparent that optimal control of gene expression and regulatory element diversity are important in plant biotechnology.
A variety of different types or classes of promoters can be used for plant genetic engineering. Promoters can be classified on the basis of characteristics such as temporal or developmental range, levels of transgene expression, or tissue specificity. For example, promoters referred to as constitutive promoters are capable of transcribing operably linked genes efficiently and expressing those genes in multiple tissues. Different types of promoters can be obtained by isolating the upstream 5′ regulatory regions of genes that are transcribed and expressed in the desired manner, e.g., constitutive, tissue enhanced, or developmentally induced.
Numerous promoters active in plant cells have been described in the literature. These include the nopaline synthase (nos) promoter and octopine synthase (ocs) promoters carried on tumor-inducing plasmids of Agrobacterium tumefaciens and the caulimovirus promoters such as the Cauliflower Mosaic Virus (CaMV) 19S or 35S promoter (U.S. Pat. No. 5,352,605), CaMV 35S promoter with a duplicated enhancer (U.S. Pat. Nos. 5,164,316; 5,196,525; 5,322,938; 5,359,142; and 5,424,200), and the Figwort Mosaic Virus (FMV) 35S promoter (U.S. Pat. No. 5,378,619). These promoters and numerous others have been used in the creation of constructs for transgene expression in plants. Other useful promoters are described, for example, in U.S. Pat. Nos. 5,391,725; 5,428,147; 5,447,858; 5,608,144; 5,614,399; 5,633,441; 6,232,526; and 5,633,435, all of which are incorporated herein by reference.
While previous work has provided a number of promoters useful to direct transcription in transgenic plants, there is still a need for novel promoters with beneficial expression characteristics. In particular, there is a need for promoters that are capable of directing expression of exogenous genes in monocotyledonous seeds during embryogenesis. Many previously identified promoters fail to provide the patterns or levels of expression required to fully realize the benefits of expression of selected genes in transgenic plants. There is, therefore, a need in the art of plant genetic engineering for novel promoters for use in monocots.